Humans have interesting limbs. Unlike in the apes, the humerus is shorter than the femur but longer than the radius. In chimps and gorillas the humerus is longer than the femur and radius. In orangutans and gibbons the humerus is longer than the femur but shorter than the radius. There are several measures anthropologists use to make these determinations. One, the humerofemoral index ([humerus x 100]/femur) measures the ratio of humerus to femur. In humans the mean of this index is (approximately) 71, in chimps 101, gorillas 116, orangutans 130 and in the gibbon 116. To compare the humerus and radius the brachial index ([radius x 100]/humerus) is used. Mean values are: humans 74, chimps 92, gorilla 80, orangutan 100 and gibbon 110. Since humans and chimps share a, relatively, recent common ancestor presumably some evolution has occured in limb proportions. We can look at the postcranial skeletons of a few hominids to determine a rough time line. Australopithecus afarensis, for example, has a humerofemoral index of about 84 (largely attributed to a smaller femur) and a brachial index of about 91 (larger radius relative to humerus). In Homo erectus, on the other hand the humerofemoral index is approximately 73 and the brachial index is 79 or roughly the same as in modern humans. So, during the course of human evolution there have been changes in forearm proportions and in femur length. For quite a while this was interpreted as an example of gradualism in hominin evolution. Then, the fly in the ointment was found.
The fly being OH62. OH62 was found in 1986 by a team led by Donald Johanson in 1986 and has been referred to H. habilis. OH62’s limb proportions were somewhat different than expected:
In people today, the upper arm bone (humerus) is considerably shorter than the upper leg bone (femur). In modern apes, the humerus and femur are nearly identical in length. Paleoanthropologists can quantify this difference using a measure called the humerofemoral index, which is defined as the length of the humerus divided by the length of the femur times 100. If the humerus and the femur are nearly equal in length, the index equals 100. If the femur is much longer than the humerus, then the index is a lower number. In OH 62, the humerofemoral index was 95; the upper arm was nearly the same length as the upper leg. For comparison, modern humans have a humerofemoral index of 70, and chimpanzees have an index of nearly 100. This means that Homo habilis had a body structure much more like an ape or Australopithecus afarensis, than a modern human. Additionally, as an adult OH 62 had only reached a stature of 1 meter or 3 feet tall. This makes OH 62 the smallest adult hominid to have ever been discovered. It also implies that OH 62 was female, and that there was a significant amount of sexual dimorphism, again, more like apes or A. afarensis (strongly dimorphic) than modern humans (not so dimorphic). Neither of these realizations were expected.
More after the pictures…
Top: Chimp, AL288-1 (Lucy), AL827-1 (Australopithecus afarensis)
Center: OH 62 (Homo habilis), Trinil 3 (Homo erectus)
Bottom: Modern human
Both indexes are high in OH62, so much so that it has been argued that it’s limb proportions are similar to the orangutan – which would mean there was a reversal in limb proportions during human evolution. Clearly something interesting is going on here – but it’s not what you would expect. In part two I will take a closer look at OH62’s femur…
Updated 03/31/2013 to fix pictures.